Surrogate decision-making by family caregivers is highly stressful and emotionally burdensome, in part because caregivers often feel unprepared to make surrogate decisions. It is not known which advance care planning (ACP) process best prepares caregivers for this role. The main purpose of this study is to evaluate whether family caregivers of patients with life-threatening illnesses are better prepared and have better experiences with surrogate decision-making: 1) when they engage in a structured ACP process together with patients (versus when patients engage in ACP alone); and 2) when they use an online decision aid for ACP (versus using standard ACP). In doing so, we will also examine: 3) interactions between these factors. Using a 2 x 2 factorial design, 200 patient/family-caregiver dyads will be enrolled in a randomized, controlled trial, and assigned to 1 of 4 groups: Group 1: Standard ACP Alone, where the patient (but not family caregiver) uses standard ACP materials to engage in ACP; Group 2: Decision Aid Alone, where the patient (but not family caregiver) uses the decision aid to engage in ACP; Group 3: Standard ACP Together, where the patient and family caregiver use standard ACP materials together; and Group 4: Decision Aid Together, where the patient and family caregiver use the decision aid together. There will be 3 study visits: Visit 1, where patients (+/- family caregivers) engage in ACP using either standard materials or the decision aid; Visit 2, where patients and family caregivers independently complete hypothetical vignettes and then caregivers are interviewed; and Visit 3, where family caregivers (but not patients) are interviewed about their experience with, and sense of preparation for, surrogate decision-making. We hypothesize that engaging in ACP Together (patient + family caregiver) will be superior to patients engaging in ACP Alone, and that using the online Decision Aid will be superior to using Standard ACP. We further anticipate that engaging in ACP Together using the Decision Aid (Group 4) will be superior to the other 3 groups with regard to family caregiver: 1) self-efficacy; 2) knowledge; 3) depth of communication about ACP; 4) agreement between their decisions on the vignettes and patients' decisions; 5) understanding of how surrogate decisions should be made; and 6) satisfaction with the ACP process. We also expect that compared to Groups 1-3, family caregivers in Group 4 will report: 7) less distress; 8) less decisional conflict; 9) greater satisfaction with decisions; and 10) better experiences with surrogate decision-making. Achieving our study aims will determine whether family caregivers who use an online decision aid to engage in ACP Together with patients who have life-threatening illnesses are better prepared and have better experiences with surrogate decision-making. If the answer is yes, as we anticipate, this research has the potential to address a critical public health concern by establishing a readily accessible, reliable, and cost- effective mechanism for family caregivers and patients to engage in advance care planning that works.